(1) Field of the Invention
This invention relates to a preparation process of an indole from an aniline and a diol. More specifically, it relates to a process for preparaing an indole by reacting an aniline with a diol in a vapor phase in the presence of a catalyst containing at least one element selected from Group Ib of the periodic table, wherein the unreacted aniline is separated from the indole, prior to, concurrently with or subsequent to being treated so as not to deteriorate the activity of the catalyst, and recycled to the reaction for its reutilization.
(2) Description of the Prior Art
Indoles are widely used as raw materials for the chemical industry such as perfumes, dyes and the like and especially, have lately been noted as a raw material for the synthesis of amino acids. Indoles have conventionally been prepared by the use of expensive raw materials or by way of long troublesome steps. However, there has recently been found a process for preparing an indole by using an aniline and a diol as economical raw materials and by way of simple steps.
As the catalyst for use in this reaction, a variety of solid acid catalysts and metallic, catalysts have been proposed. The present inventors have examined the reaction from various angles. As a result, it has become clear that a catalyst containing any one of Cu, Ag and Au, which are the elements of Group Ib of the periodic table, is effective for the reaction and that where an indole is synthesized from an aniline and a diol by using this catalyst, it is necessary to cause a large amount of the aniline to exist in the reaction system in order to obtain the indole in high yield. Therefore, where this catalyst is used to synthesize the indole, it is indispensable to separate and recover a large amount of the aniline contained in the reaction mixture and use it repeatedly as a raw material for the reaction.
When the reaction is effected while using repeatedly such recovered aniline for a long period of time, the catalyst will exhibit a reduction in its activity to a degree that depends on the type of catalyst and the reaction conditions, as as compared with the case in which are unused aniline is employed.
Specifically, even a catalyst, which is capable of maintaining its catalytic activity at a sufficiently high level over a period of hundreds of hours where the reaction is effected without recycling the recovered aniline, will exhibit a marked reduction in its catalytic activity when the reaction is carried out by feeding the recovered aniline while maintaining the other conditions entirely identical. The catalyst thus-lowered in its catalytic activity will have to be subjected to frequent regeneration treatments by heating it in an oxygen atmosphere. This regeneration treatment is troublesome and its excessive repetition may sometimes impair the economical efficiency of the process. The reason of the reduction of its catalytic activity has not been clarified in detail. However, since the catalyst can be regenerated and activated by conventional means, it is assumed as the main reason that certain organic substances are converted to carbonaceous materials under the reaction conditions to precipitate on the surface of the catalyst and cover its active sites. The reduction of the activity can be minimized by carrying out the reaction in an atomosphere of hydrogen or by adding water to the reaction system. However, their effects are not necessarily satisfactory. This leads to the assumption that the recovered aniline may contain substances which are poisonous to the catalyst.
The liquid reaction product obtained by condensing a gaseous reaction mixture withdrawn from a reactor contains an indole produced, water, an unreacted diol, an excess aniline and still further small amounts of various by-products. Although some of the by-products are separated and identified, a majority of them have not yet been clarified in their chemical structures, physico-chemical properties, etc. However, it is assumed that these unclarified by-products may involve substances which are recycled to the reactor without being removed from the recovered aniline and precipitate on the surface of the catalyst as carbonaceous materials, thereby causing the activity of the catalyst to deteriorate. These by-products are assumed to be the ones which are difficult to be separated from the recovered aniline by usual distillation, in view of the fact that the aniline recovered by distillation in a conventional manner exerts an adverse effect upon the activity of the catalyst.